It is known that media which are anisotropic in shape can form liquid-crystalline phases, known as mesophases, on warming. The individual phases differ through the spatial arrangement of the major parts of the molecules on the one hand and through the molecular arrangement with respect to the long axes on the other hand (G. W. Gray, P. A. Winsor, Liquid Crystals and Plastic Crystals, Ellis Horwood Limited, Chichester, 1974). The nematic liquid-crystalline phase is distinguished by the fact that there is only one alignment long-distance ordering due to the long molecular axes lining up in parallel. Under the prerequisite that the molecules making up the nematic phase are chiral, a cholesteric phase forms, in which the long axes of the molecules form a helical superstructure perpendicular thereto (H. Baessler, Festkxc3x6rperprobleme XI, 1971). The chiral moiety may be present either in the liquid-crystalline molecule itself or added to the nematic phase as a dope inducing the cholesteric phase. This phenomenon was first studied on cholesterol derivatives (eg. H. Baessler, M. M. Labes, J. Chem. Phys. 52 (1970) 631; H. Baessler, T. M. Laronge, M. M. Labes, J. Chem. Phys. 51 (1969) 3213; H. Finkelmann, H. Stegemeyer, Z. Naturforschg. 28a (1973) 799; H. Stegemeyer, K. J. Mainusch, Naturwiss. 58 (1971) 599; H. Finkelmann, H. Stegemeyer, Ber. Bunsenges. Phys. Chem. 78 (1974) 869).
The cholesteric phase has remarkable optical-properties: large optical rotation and pronounced circular dichroism caused by selective reflection of circular-polarized light within the cholesteric layer. The different colors to be observed depending on the viewing angle depend on the pitch of the helical superstructure, which is itself dependent on the twisting power of the chiral component. The pitch and thus the wavelength range of the selectively reflected light of a cholesteric layer can be varied, in particular, by changing the concentration of a chiral dope. Such cholesteric systems offer interesting opportunities for practical use. Thus, incorporation of chiral moieties into mesogenic acrylic esters and alignment in the cholesteric phase, for example after photocrosslinking, can give a stable, colored network, but the concentration of the chiral component therein cannot be changed (G. Galli, M. Laus, A. Angeloni, Makromol. Chem. 187 (1986) 289). Admixing of non-crosslinkable, chiral compounds with nematic acrylic esters can give by photocrosslinking a colored polymer which still contains high proportions of soluble components (I. Heyndricks, D. J. Broer, Mol. Cryst. Liq.
Cryst. 203 (1991) 113). Furthermore, random hydrosilylation of mixtures of cholesterol derivatives and acrylate-containing mesogens using defined cyclic siloxanes followed by photopolymerization allows the production of a cholesteric network in which the chiral component can comprise up to 50% of the material employed; however these polymers also contain significant amounts of soluble components (F. H. Kreuzer, R. Maurer, C. Mxc3xcller-Rees, J. Stohrer, Paper No. 7, 22nd Freiburg Congress on Liquid Crystals, Freiburg 1993).
DE-A-35 35 547 describes a process in which a mixture of cholesterol-containing monoacrylates can be converted into cholesteric layers by photocrosslinking. However, the total proportion of the chiral component in the mixture is about 94%. Although a material of this type is not very mechanically stable as a pure side-chain polymer, an increase in the stability can, however, be achieved by means of highly crosslinking diluants.
In addition to the nematic and cholesteric networks described above, smectic networks are also known; these are prepared, in particular, by photopolymerization/photocrosslinking of smectic liquid-crystalline materials in the smectic liquid-crystalline phase. The materials used for this purpose are generally symmetrical liquid-crystalline bisacrylates, as described, for example, by D. J. Broer and R. A. M. Hikmet, Makromol. Chem., 190, (1989), 3201-3215. However, these materials have very high clearing points of  greater than 120xc2x0 C., with the associated risk of thermal polymerization. If an Sc phase exists, piezoelectric properties can be achieved by admixing chiral materials (R. A. M. Hikmet, Macromolecules 25, (1992), p. 5759).
The present invention relates to structures of the formula I
X[xe2x80x94Y1xe2x80x94A1xe2x80x94Y2xe2x80x94Mxe2x80x94Y3xe2x80x94A2xe2x80x94Z]nxe2x80x83xe2x80x83I,
where
X is a silicon-free, n-valent central unit, the radicals
A1 and A2, independently of one another, are a direct bond or a spacer, the radicals
Y1, Y2 and Y3, independently of one another, are a direct bond, O, S, CO, OCO, COO, OCOO, 
M is a mesogenic group,
Z is a polymerizable group, and
n is a number from 2 to 6, where
R is hydrogen, or C1- to C4-alkyl, and the combination of Mxe2x80x94Y3 A2xe2x80x94Z can be a cholesteryl radical.
The radicals X can be aliphatic, aromatic or cycloaliphatic and may additionally contain heteroatoms. Also suitable are divalent elements and groups such as O, S, SO2 and CO.
X can be, in particular, C2- to C12-alkylene, -alkenylene or -alkynylene radicals, which may be interrupted by one or more O or S atoms or NR groups, or phenylene, benzylene or cyclohexylene or radicals of the formula 
Examples of individual alkylene, alkenylene or alkynylene radicals are, 
where
p is a number from 2 to 12,
q is a number from 1 to 3 and
r is a number from 1 to 6.
Suitable spacers are all groups which are known for this purpose; the spacers are usually linked to X via ester or ether groups or a direct bond. The spacers generally contain from 2 to 30, preferably from 2 to 12, in particular from 6 to 12, carbon atoms and may be interrupted in the chain, for example by O, S, NH or NCH3. Suitable substituents for the spacer chain are fluorine, chlorine, bromine, cyano, methyl or ethyl.
Examples of representative spacers are the following: 
where
p and q are as defined above.
Y1, Y2 and Y3 are preferably a direct bond, O, OCO, COO or OCOO.
The radicals M can in turn be the known mesogenic groups, in particular radicals containing aromatic or heteroaromatic groups. The mesogenic radicals conform, in particular, to the formula II
(xe2x80x94Txe2x80x94Y1)sxe2x80x94Txe2x80x83xe2x80x83II,
where the radicals
T, independently of one another, are cycloalkylene, heterocycloalkylene, aromatic or heteroaromatic radicals, the radicals
Y1 independently of one another are O, COO, OCO, CH2O, OCH2, CHxe2x95x90N or Nxe2x95x90CH or a direct bond, and
s is from 1 to 3.
s is preferably 1 or 2.
Y1 is preferably xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94 or a direct bond.
The radicals T are generally carbocyclic or heterocyclic aromatic ring systems, which may be substituted by fluorine, chlorine, bromine, cyano, hydroxyl or nitro, conforming, for example, to the following basic structures: 
The mesogenic groups M are particularly preferably the following, for example: 
Preferred groups Z are those which can be polymerized by a photochemical initiation step, i.e. in particular groups of the structure: CH2xe2x95x90CHxe2x80x94, CH2xe2x95x90CCl, CH2xe2x95x90C(CH3)xe2x80x94 or 4-vinylphenylyl. Preference is given to CH2xe2x95x90CHxe2x80x94, CH2xe2x95x90CClxe2x80x94 and CH2xe2x95x90C(CH3)xe2x80x94, particular preference being given to CH2xe2x95x90CHxe2x80x94 and CH2xe2x95x90C(CH3)xe2x80x94.
General methods for the preparation of the compounds of the formula I are known from the literature, for example the reaction with dicyclohexylcarbodiimide (DCC) for the preparation of esters. Details on the reactions are given in the examples, where parts and percentages are by weight, unless stated otherwise.
The compounds of the formula I are liquid-crystalline and can form smectic, nematic or cholesteric phases, depending on the structure. They are suitable for all purposes for which liquid-crystalline compounds are usually used.
Novel compounds adopt an intermediate position between low-molecular-weight and polymeric liquid-crystalline compounds. In contrast to polymers, they can be prepared reproducibly, have substantially uniform structures and nevertheless have viscosities similar to those of polymers.
In order to establish desired properties, it may be expedient to use mixtures of compounds of the formula I or mixtures with other liquids, it being possible for these mixtures to be prepared in situ or by mechanical mixing.
The novel compounds are particularly suitable as alignment layers for liquid-crystalline materials, as photocrosslinkable adhesives, as monomers for the production of liquid-crystalline networks, as base material for the preparation of polymerizable liquid-crystal systems which can be doped by chiral compounds, as polymerizable matrix monomers for polymer-dispersed displays or as base material for polymerizable liquid-crystalline materials for optical components, such as polarizers, retardation plates or lenses. They are furthermore suitable in combination with low-molecular-weight, polymerizable liquid-crystalline compounds as film formers.
The melting points in the examples were determined by means of a polarizing microscope. Temperature control was effected on a Mettler FP80/82 microscope heating stage.